Applicant has earlier proposed in his United States Letters Patents Nos. 3,557,955 and 3,550,774 (herein incorporated by reference) a new and novel type of "deep bed" filter wherein synthetic polymeric materials in finely granulated form are utilized as the filter media.
The utilization of synthetic polymeric materials, such as polyvinyl chloride (PVC) materials, was preferred because of the combination of the necessary physical characteristics of such materials. For example, the method and apparatus disclosed in U.S. Pat. No. 3,550,774 cleanses the filter medium by placing a mixture of medium and earlier removed contaminants in a slurry and then subjecting this slurry to a violent washing action, i.e., flowing the slurry past a perforate scrubbing element. Any practical filter medium for utilization in such a method must be light (preferably having a specific gravity of 1.55 or less), strong (having a modulus of elasticity of 40,000 PSI or greater) and non-abrasive (so as not to wear away the perforate scrubbing element).
These requirements of the filter medium eliminates from consideration the common granular filter media, such as sand, anthracite, and activated carbon. Many other materials, particularly synthetic polymeric materials are eliminated because of their high cost.
A separate and distinct problem in the art of filtration or liquid separation is the removal of oily industrial wastes from a water or non-water mixture. For example, oily wastes are typically removed from industrial plant discharges by the use of chemicals, gravity settling and other conventional techniques. These methods, however, are generally incapable of reducing the oil content in discharge liquids to below about 150 ppm. To achieve better clarity, large clarifiers are required, and even then, it is somewhat difficult to consistently achieve an extremely clean liquid having an oil content lower than about 30 ppm.
A further method of separating oil comtaminants from a water based solution is disclosed in U.S. Pat. No. 3,526,153 to Tully. This patent suggests treating certain oil-absorbent materials with colloidal hydrophobic metal or metalloid oxides to thereby form an improved oil-absorbent composition. Within the long list of examples of possible oil-absorbent materials, this patent includes a broad suggestion of treated "nut shells," wherein the coating is relied upon to attract the oil. This patent disclosure fails in several material respects, including a suggestion for rejuventating and reusing the filter media.
Applicant has found that a very specific "nut shell," i.e., granulated black walnut shells, possesses a capability of coalescing oil during contaminant liquid flow and accumulating the coalesced oil in the interstices of the filter media bed. Black walnut shells also exhibit a relatively weak affinity for oil enabling rejuvenation of the bed by conventional backwash cycles. The economic benefits flowing from the present invention will be apparent to one skilled in the art of filtration or liquid separation, in view of: (1) the commercial availability of black walnut shells; (2) the monetary savings from the elimination of medium treatment; (3) the relative ease of removing the accumulated oil from the medium; and (4) the ability to reuse the filter media.
Accordingly, many natural materials have been investigated for (1) their strength to withstand the slurrying and violent cleansing action when employed in combination with the scrubber tube of U.S. Pat. No. 3,550,774; and (2) their affinity for attracting and accumulating oils suspended in an emulstion, and then for releasing the accumulated oils during rejuvenation of the filter bed.